A so-called resin-laminated board has been employed as an interior material of an automobile, a building material, and a distributing and packaging material. A resin-laminated board has a front-side material made of resin and a backside material made of resin. The backside material of the resin-laminated board has a recess, and an end of the recess is butted on an inner surface of the front-side material. In particular, in the case of the interior material of an automobile and the building material, whose appearance is considered important, a nonwoven fabric is pasted on a front side of the front-side material. A variety of methods has been conventionally suggested for fabricating such a resin-laminated board. A first method employs a technique of fabricating a resin-laminated board through integral extruding blow molding with the use of molten resin. The resin-laminated board fabricated by this method has a recess for connecting the front-side material and the backside material. Thus, the stiffness (especially, the compression stiffness of a planar material relative to the load in the vertical direction) can be secured, compared with a hollow double-wall structure simply having a hollow part inside.
A second method employs a technique as disclosed in Patent Document 1. In this technique, a pair of rollers is used. One roller has a number of protrusions on a surface thereof. These protrusions are disposed in a houndstooth pattern on the surface of the roller. Two molten sheets extruded separately are pressed with a predetermined pressing force between the pair of rollers, thereby forming a recess in one sheet. The two sheets are welded in a state that the bottom of the recess is butted on an inner surface of the other sheet. Moreover, another sheet is welded on a surface of the one sheet that is provided with an opening of the recess. As a result, the sheet is welded on the front side and the backside of the sheet having the recess, and a three-layer structure is obtained.
A third method employs a technique as disclosed in Patent Document 2. Unlike the second method, in this technique, a number of protrusions are disposed in a houndstooth pattern on each surface of a pair of rollers. Two molten sheets extruded separately are pressed with a predetermined pressing force between the pair of rollers. On this occasion, the two sheets are welded to each other so that the recess is formed in each sheet and so that the bottoms of the corresponding recesses of the sheets are welded to each other. Further, other sheets are welded on the surfaces of the sheets on the opposite side. As a result, the sheets are welded on the surfaces of the sheets that are provided with the opening of the recess, and a four-layer structure is obtained. The technique as above for fabricating the resin-laminated board by extruding the molten sheets has technical problems as below.
One problem is that it is difficult to obtain a resin-laminated board with sufficient strength with no directivity while securing high fabrication efficiency. More specifically, the problem peculiar to the extrusion-molding common among the first and third methods is that the end processing such as a thermal sealing process is necessary because the end of the sheet in the extruding direction becomes open. This extra step deteriorates the fabrication efficiency as a whole. As another problem, it is difficult to secure enough welding time for welding the sheet provided with the recesses in a houndstooth pattern and the other sheet or welding the sheets provided with the recesses in a houndstooth pattern. This is because the welding is performed merely by the pressing force when the sheet is fed by the pair of rollers and conveyed between the rollers. Therefore, because of the insufficient welding, it is difficult to secure enough strength as the resin-laminated board. This leads to deterioration in quality. Such problems can be avoided by employing the blow molding instead of continuous extrusion molding. The blow molding can avoid the deterioration in fabrication efficiency due to the end processing as above and avoid the lack of strength due to the insufficient welding. Patent Document 3 discloses a method in which such blow molding is employed. Patent Document 3 discloses that a recess is provided for a backside wall (an end of the recess is butted on, and welded to the inner surface of a front-side wall) using a tubular parison in a molten state and that a mounting material is pasted on an outer surface of the front-side wall. However, the blow molding for performing the molding while a blowing pressure is applied using the tubular parison causes another technical problem. That is, the molding performed while a blowing pressure is applied using a tubular parison with uniform thickness in a circumferential direction makes it difficult to sufficiently reduce the weight and the thickness of the resin-laminated board.
More specifically, the tubular parison is generally extruded from an annular slit between die cores. Therefore, the tubular parison has a substantially constant thickness in the circumferential direction. Moreover, when a pair of split molds is clamped, the blowing pressure is applied from a sealed space in the mold. Thus, the pressing force relative to the mold of the parison is uniform across the entire surface of the parison. The parison pressed by one mold that forms the recess, however, produces a locally thin part due to the extension of the parison depending on the depth of the recess and the blow ratio based on the opening diameter. Meanwhile, such a thin part is not produced in the other mold that does not form the recess. In view of this, the thickness of the tubular parison needs to be set in accordance with the thin part of the one mold. This results in the formation of a sheet with extra thickness for the other mold. Thus, in the case of using the tubular parison with a substantially constant thickness in the circumferential direction, the difference in thickness is generated inevitably between the wall surface having a plurality of recesses after the blow molding and the wall surface not having the recess. Therefore, the sufficient reduction in weight and thickness of the resin-laminated board cannot be achieved. In this point, Patent Document 4 discloses a method of fabricating a plate-shaped body of a thermoplastic resin. In this method, a plate-shaped body is formed using two molten sheets based on the tubular parison. This plate-shaped body has a hollow part inside. Two surfaces of the plate-shaped body that face each other are provided with a plurality of recesses with the bottoms being adjacent to each other. However, it is preferable to avoid that one sheet is provided with a number of recesses with the openings thereof facing frontward. This is because, especially in the case of the resin-laminated board for the interior material for an automobile or a building material whose appearance is considered important, one sheet on the front side needs to have a decorative material, such as a nonwoven fabric, pasted thereon.